Add serviceNameOverride to docs

[vepatel]

• serviceNameOverride needs to be add to helm config table in official docs https://docs.nginx.com/nginx-ingress-controller/installation/installation-with-helm/#configuration
• fix usage from --set controller.serviceNameOverride to --set serviceNameOverride

[github-actions]

Hi @vepatel thanks for reporting!

Be sure to check out the docs and the Contributing Guidelines while you wait for a human to take a look at this 🙂

Cheers!